The effect of thermal annealing on layers of CuInS2 nanocrystals (NCs) stabilized with (NH4)2S was investigated using in situ transmission electron microscopy (TEM), in situ X-ray diffraction (XRD), thermogravimetric analysis combined with mass spectrometry (TGA-MS) and X-ray photoelectron spectroscopy (XPS). It is shown that these inorganic, chalcogen containing ligands inhibit NC sintering up to 450 °C in an inert atmosphere. On the other hand, sintering can be promoted by annealing in hydrogen gas. A similar behavior is found with Cu2ZnSnSe4 and CdSe NCs. We attribute the inhibited sintering to the oxidation of the S2− originally stabilizing the NCs to sulfite or sulfate moieties, where oxidation is possible either by exposure of the films to air or by thermal decomposition of residual solvent molecules present in the film under inert conditions.

The micropatterning of layers of colloidal quantum dots (QDs) stabilized by inorganic ligands is demonstrated using PbS core and CdSe/CdS core/shell QDs.A layer-by-layer approach is used to assemble the QD films, where each cycle involves the deposition of a QD layer by dipcoating, and the replacement of the native organic ligands by inorganic moieties, such as OH - and S 2-, followed by a thorough cleaning of the resulting film.This results in a smooth and crack-free QD film on which a photoresist can be spun.The micropatterns are defined by a positive photoresist, followed by the removal of uncovered QDs by selective wet etching with an HCl/H 3 PO 4 mixture.The resulting patterns can have submicron feature dimensions, limited by the resolution of the lithographic process, and can be formed on planar and 3D substrates.It is shown that the photolithography and wet etching steps have little effect on the photoluminescence quantum yield of CdSe/CdS QDs.Compared with the unpatterned CdSe/ CdS QD film, only a 10% degradation in the quantum yield is observed.These results demonstrate the feasibility of the proposed micropatterning method to implement the large-scale device integration of colloidal quantum dots.

We link the extent of Pb for Cd cation exchange reactions in PbS colloidal quantum dots (QDs) to their surface chemistry. Using PbS QDs with either a full or a partial surface coverage by excess Pb, we demonstrate the central role played by vacant cation sites on the QD surface. They facilitate the adsorption of cations from solution, and they act as a source of vacancies needed for the transport of cations through the crystal lattice. This model explains our finding that the cation exchange reaction runs to completion when using a low Cd excess in the exchange bath, while it is impeded by a high Cd excess. Whereas in the latter case, the QD surface is poisoned by surface Cd, the former conditions provide the mixture of surface Cd and vacant surface sites the exchange reaction needs to proceed. This understanding provides a missing link needed to build a unifying mechanistic picture of cation exchange reactions at nanocrystals.

HgTe is semi-metallic as a bulk material but when the crystal dimensions are reduced to the nanometer range, a band gap or HOMO-LUMO gap that is tunable throughout the IR region by the nanocrystal size opens up. The IR contains a number of technologically important wavelength ranges such as the 1300-1600 nm window used for telecommunication and the mid IR used for infrared spectroscopy. For these applications, HgTe quantum dots could be used for the emission, detection or modulation of light. In this study, we discuss a novel synthesis for HgTe QDs. Using XRD and TEM, we show that the synthesis results in HgTe nanocrystals with the zincblende crystal structure and diameters of a few nanometer. Moreover, the HgTe QDs exhibit a clear absorption onset and a narrow photoluminescence spectrum corresponding to the quantum confined band gap transition. A detailed study of the reaction yield and the nanocrystal size as a function of time provides the insight in the reaction mechanism needed to tune the nanocrystal size and, concomitantly, the wavelength of absorption onset and photoluminescence. As a result, HgTe QDs with a spectrally narrow emission in the 1000 to 2500 nm range could be obtained. Our findings thus provide a thorough understanding of the mechanism of HgTe QD synthesis and result in materials that can be readily explored for applications such as photo detection and IR light emission.

We describe the synthesis of metal selenide nanocrystals, including CdSe, ZnSe, CuInSe2 and Cu2(Zn,Sn)Se4, by the hot injection of selenium powder dispersed in a carrier solvent. Since this results in a fast and high yield nanocrystal formation, we argue that the approach is well suited for the low cost, large volume production of nanocrystals.

The synthesis of metal selenide nanocrystals based on the injection of selenium powder in a hot mixture containing the metal cation precursor complexed by a carboxylic acid is demonstrated by means of the formation of CdSe and ZnSe nanocrystals. In both cases, the synthesis can reach reaction yields of 80-85% within 5 min. In the case of CdSe nanocrystals, a more extensive study shows that even without protective atmosphere, the synthesis leads to state-of-the-art nanocrystals with low size dispersion. Importantly, the size of the nanocrystals at close to full yield can be changed by varying the carboxylic acid chain length, whereas the solid loading, that is, the amount of nanocrystals formed over the reaction volume, of the synthesis can be the 10-fold of typical literature syntheses. The potential of this reaction for the scaled up production of metal selenide nanocrystals is discussed and supported by the automated, parallel synthesis of CdSe nanocrystal batches using this heterogeneous selenium precursor where the standard deviation on the nanocrystal diameter is less than 1.5%.

In the field of fluorescent semiconductor quantum dots (QDs), alloyed QDs open up new possibilities and opportunities. Indeed, these systems allow to tune the optical properties of the nanocrystals without changing their size. This is of particular interest for the integration of the QDs in devices such as LEDs or for their use as biological labels. We recently developed a novel method for the synthesis of CdSe and ZnSe binary QDs in colloidal solutions that is fast and highly efficient. This method is based on a heterogeneous Se-ODE precursor consisting of a simple dispersion of Se powder (200 mesh) in octadecene (ODE) and showing very high reactivity towards Cd precursor. In this contribution we will demonstrate that this method can be extended to the synthesis of CdSe1-xSx homogeneously alloyed QDs (0 ≤ x ≤ 1).

Semiconductor quantum dots (QDs) are luminescent nanocrystals offering bright and narrow emissions from UV to near-infrared. As a result, they find numerous applications as light emitters in a wide range of technologies, from photonics to biotechnologies. However, tuning the emission color is usually done by varying the size of the nanocrystals, what can be problematic for their integration in electronic and optoelectronic devices. To overcome this limitation, the syntheses of alloyed QDs, such as ternary Cd(Se,Te), have been developed in the past years. These systems allow a fine tuning of the emission color by varying the composition of the alloy without changing the size of the QDs. We recently developed a new approach for the synthesis of CdSe and ZnSe QDs based on a heterogeneous Se precursor. This fast synthesis with high reaction yield allows the production of high quality nanocrystals at large scale. In this contribution, we will show how this new approach can be extended to the synthesis of homogeneously alloyed Cd(Se,S), Zn(Se,S) but also (Cd,Zn)(Se,S) QDs thanks to the equivalent reactivity of the Se and S precursors towards the Cd and Zn precursors. The optical properties of the alloyed QDs will be fully described, with a particular attention to the dependence of the band gap on their composition. Finally, the interest of tuning the bang gap for the design of new heteronanostructures will also be highlighted through the growth of shells on the alloyed QDs.

The absorption cross section of colloidal quantum dots in close-packed monolayers shows a 4 (CdSe) to 5-fold (PbS) enhancement compared to quantum dots in a dilute dispersion. Quantitative agreement is demonstrated between the value and the size dependence of the enhancement and theoretical model predictions based on dipolar coupling between neighboring quantum dots. This collective optical behavior offers a new degree of freedom in the custom design of optical properties for electro-optical devices.

Following their increasing use, the supply of larger quantities of monodisperse colloidal nanocrystals(NC) necessitates a scaling up of their production. As a result, synthesis cost, tuneability of the NC size at full yield and synthesis reproducibility have become key issues. Finding an optimal approach in this respect is a matter of methodology, involving the use of larger scale or automated batch reactors or continuous flow-line approaches, yet it also concerns a reassessment of the reagents used and the reaction conditions needed. This is especially true for selenium precursors used to synthesize metal selenide nanocrystals such as CdSe. These involve selenium dissolved in either tri-octylphosphine (TOP) [1]- which is expensive and oxygen sensitive - or 1-octadecene, which leads to a precursor (homogeneous ODE-Se) with a low reactivity and a limited reaction yield [2]. Here, we propose an alternative approach to synthesize metal selenide NC’s using a Se precursor that adds a high reactivity to the advantages of homogeneous ODE-Se. The method involves the direct injection of a heterogeneous mixture of selenium powder dispersed in a carrier liquid in a hot solvent containing a metal carboxylate as the cation precursor and excess carboxylic acid. Both in the case of cadmium and zinc carboxylates, we find that the injection of this heterogeneous ODE-Se precursor is followed by the formation of monodisperse nanocrystals, reaching chemical yields up to 80-90% within a few minutes. Moreover, the reaction can be run under ambient conditions without compromising the quality of the end product and the NC diameter reached at close to full yield can be tuned by changing the carboxylic acid chain length. In addition, since the amount of selenium injected is not limited by the solubility of selenium, the reaction can be executed with a high solid load, thus minimizing the amount of solvent needed. Finally, we demonstrate that syntheses involving the heterogeneous Se precursor as proposed here can be reproducibly executed on an automated synthesis platform, thus showing the potential of this novel approach for scaling up the production of colloidal metal selenide nanocrystals. [1] J. Am. Chem. Soc., 2001, 123 (1), pp 183-184 [2] J. Phys. Chem. B, 2005, 109 (44), pp 20665-20668

We report the experimental study of hole transport in poly(vinylcarbazole) (PVK) films doped with colloidal CdSe/ZnS core-shell quantum dots (QDs) determined using the Time-of-Flight (TOF) method. The miscibility between PVK and the QDs is improved by capping the QDs with a novel 11-(N-carbazolyl) undecanoic acid (C11) ligand instead of commonly used organic ligands, such as oleic acid. The study of the hole mobility of the pristine and doped PVK films with a doping concentration of the C11 capped QDs ranging from 1.61 x 10(17) to 7.10 x 10(18) dots/cm(3) was performed as a function of electric field and temperature in the range of 10(5)-10(6) V/cm and 298-338K, respectively. Upon increasing the QD concentration, a decrease of hole mobility was observed by up to nearly 2 orders in magnitude at a doping concentration of 3.87 x 10(18) dots/cm(3) at T = 298 K. This suggests that the QDs induce shallow hole traps. The field and temperature dependence of the hole mobility was characterized using the Bassler disorder model and showed an increase of the energetic disorder (sigma) from 124 to 204meV as well as of the spatial disorder (Sigma) from 0.95 to 5 when the concentration of the QDs was increased to 3.87 x 10(18) dots/cm(3). At higher concentration of the QDs (7.10 x 10(18) dots/cm(3)), an increase of the hole mobility was observed suggesting hopping of the holes through the QD clusters. In addition, we also found that for this high doping concentration, the field dependence of the hole mobility was no longer in agreement with the Bassler disorder model. One should consider that at this doping concentration, the volume occupied by the inorganic (CdSe + ZnS) and organic (C11) components of the QDs in the doped film was estimated to be 14.6 and 15.8 volume %, respectively. This implies that the volume fraction of the inorganic material is very close to the percolation threshold, which amounts to 17 volume % for small spherical particles embedded in a three dimensional matrix. Furthermore, the conductivity data suggest a qualitative change in film properties between the samples with 3.87 x 10(18) and 7.10 x 10(18) dots/cm(3). The study of film morphology by atomic force microscopy (AFM) experiment shows that while for the film with 3.87 x 10(18) dots/cm(3) the surface of the film has still the same features as that of a pristine PVK film, this is no longer the case for the film with 7.10 x 10(18) dots/cm(3), where shallow holes with a diameter of 100 to 200 nm become visible. These holes with the size much larger than the diameter of an individual QD likely correspond to clusters of the QDs. Upon further increasing the QD concentration to 9.68 x 10(18) dots/cm(3), the density of these holes is also increased. A correlation between the conductivity data and the film morphologies indicates that the presence of these QD clusters in the sample with 7.10 x 10(18) dots/cm(3) does not only change the homogeneity and roughness of the film but also leads to a significant change in the shape of the density of states of the energy sites for hopping holes resulting in a field and temperature dependence of the hole mobility that is no longer compatible with the Gaussian disorder model for this sample.Furthermore, the presence of these "hole" structures observed with AFM might imply a formation of large QD clusters in the polymer film, which form continuous pathways for charge carrier hopping between the opposite electrodes. (c) 2013 AIP Publishing LLC.

We investigate colloidal Fe(3)O(4) nanocrystals as a catalyst system for carbon nanotube (CNT) growth that allows for decoupling the CNT growth step from the catalyst shaping and activation step. The system consists of 6.4 nm Fe(3)O(4) nanocrystals synthesized using a solution-based thermal decomposition reaction and, subsequently, transferred as hexagonally ordered Langmuir-Blodgett (LB) monolayers on TiN substrates. We demonstrate for the first time aligned CNT growth from LB deposited nanocrystals on a metallic underlayer. The hexagonally ordered monolayers of catalyst particles show promising stability up to the CNT growth temperature. In situ TEM heating experiments were performed to find this onset of particle deformation and showed stability of the nanoparticles up to 600 °C. The particle coalescence at high temperatures was also evidenced by the increasing CNT diameter, from 9.5 nm at 580 °C to 16 nm at 630 °C. By choosing to work at temperatures below the onset particle coalescence temperature, equivalent CNT diameters were obtained under different catalyst activation and growth conditions. The high stability of the catalyst on the metallic underlayer enables us to study CNT growth kinetics independently of the catalyst shaping step. This work opens a route towards combining growth studies with an electrical evaluation of the CNT growth as the TiN can be used as the bottom contact.

In the field of light-emitting devices, solar cells and photodetectors, inorganic colloidal nanoparticles are very promising because of their interesting physical and chemical properties. One drawback of these materials is that in order to keep them stable in solution they need to be stabilized with surface ligands. These surface ligands are for example molecules with long hydrocarbon chains (C8-C18). As a result, the nanoparticles are surrounded by a highly insulating barrier that makes their use in above devices problematic. One way of getting rid of this problem is exchanging these long ligands with shorter ones or with inorganic molecules such as molecular Metal Chalcogenide Complexes (MCCs). They keep the nanoparticles stabilized in the colloid and enable strong electronic coupling. An interesting MCC ligand is for example Sn2S64-. However, when this MCC solution is synthesized, also other Sn-complexes (f.e. SnS44-) can be formed. The lack of hydrogen atoms in these systems makes them challenging to characterize, moreover, literature concerning these systems is scarce. Using 119Sn NMR we attempted a full characterization of the solutions of MCC complexes as synthesized.